Polymer Amphiphiles for Photoregulated Anticancer Drug Delivery

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Biological and Medical Applications of Materials and Interfaces

Polymer Amphiphiles for Photoregulated Anticancer Drug Delivery Valentina Brega, Federica Scaletti, Xianzhi Zhang, Li-Sheng Wang, Prudence Li, Qiaobing Xu, Vincent M. Rotello, and Samuel W. Thomas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18099 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on January 7, 2019

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ACS Applied Materials & Interfaces

Polymer Amphiphiles for Photoregulated Anticancer Drug Delivery Valentina Brega,a Federica Scaletti,b Xianzhi Zhang,

b

Li-Sheng Wang,b

Prudence Li,c Qiaobing Xu,c Vincent M. Rotellob and Samuel W. Thomas III*

a

a: Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford MA 02155, United States.

b: Department of Chemistry, University of Massachusetts Amherst, 710 Nt. Pleasant Street, Amherst MA 01003, United States.

c: Department of Biomedical Engineering, Tufts University, 4 Colby Street, Medford MA 02155, United States.

KEYWORDS Self-assembly, light-responsive materials, amphiphilic polymers, drug delivery, doxorubicin.

ACS Paragon Plus Environment

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ABSTRACT We report the synthesis of amphiphilic polymers featuring lipophilic stearyl chains and hydrophilic poly(ethylene glycol) (PEG) polymers that are connected through singlet oxygen-cleavable alkoxyanthracene linkers. These amphiphilic polymers assembled in water to form micelles with diameters of ~20 nm. Reaction of the alkoxyanthracene linkers with light and O2 cleaved the ether C-O bonds, resulting in formation of the corresponding 9,10-anthraquinone derivatives and concomitant disruption of the micelles. These micelles were loaded with the chemotherapeutic agent doxorubicin, which was efficiently released upon photo-oxidation. The drug-loaded reactive micelles were effective at killing cancer cells in vitro upon irradiation at 365 nm, functioning through both doxorubicin release and photodynamic mechanisms.

Introduction Materials that degrade upon application of specific chemical or physical triggers have potential for on-demand delivery,1 and other applications.2,3 A range of responsive linkers cleave upon application of a specific endogenous stimulus, such as disulfides for chemical reduction4 or acetals for hydrolytic


2

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degradation.5

Photodegradable

materials

provide

an

external

stimulus

for

activation, harnessing the unique advantages of light, including spatiotemporal precision and the ability to penetrate through barriers impermeable to traditional chemical

reagents.

Photocleavable

groups

are

common

components

of

photodegradable macromolecular architectures,6,7 such as UV-disruptable block copolymer micelles that release Nile Red upon irradiation.8

Furthermore, a

number of approaches are applicable to drug delivery systems that respond to visible

or

NIR

light,9

such

as

two-photon

absorption,10,11

photothermal

heating12,13,14 and upconverting nanoparticles.15,16,17,18,19 As an alternative, the reactive oxygen species (ROS) singlet oxygen (1O2) is both readily generated photochemically and can cleave specific chemical bonds. 1O

2

engages in [2+2] cycloadditions and ene reactions, and can also act as a

dienophile, oxidizing anthracenes and other polycyclic aromatic hydrocarbons to endoperoxides through [4+2] cycloaddition reactions.20,21,22 The cytotoxic nature of

1O

2

makes it important in photodynamic therapy, a treatment that uses a

photosensitizer to produce ROS upon irradiation to destroy cancer cells.23 The


3


Page 4 of 34

ubiquity and ease of generation of 1O2 has led to the concept of 1O2 cleaving chemical bonds selectively. Beyond the initially reported electron-rich alkenes, other moieties have emerged that undergo bond cleavage upon reaction with 1O

2,

such as aminoacrylates,24,25,26,27,28 alkoxyanthracenes,29,30,31,32,33,34,35 and

thioketals.36,37 These classes of 1O2-cleavable groups have been integrated into a

range

of

degradable

materials,

including

block

copolymer

micelles,38,39

nanoparticles,40 nanorods,41 and vesicles.42 Two principal methods exist that harness

1O

2-responsive

linkers for applications in drug delivery: i) materials in

which a prodrug is covalently linked through a

1O

2-clevable

materials in which the drug is non-covalently trapped in a

linker,43 and ii) 1O

2-degradable

carrier.39 Cleavage of linkages between hydrophobic and hydrophilic segments of amphiphiles provides an effective strategy to disrupt micelles.39 Herein we report

1O

2-cleavable

amphiphiles featuring alkoxyanthracene derivatives that

bridge lipophilic and hydrophilic segments of polymers that assemble into photooxidatively degradable micelles. We prepared two amphiphilic polymers


4

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(9,10-C18PEG and 3,9-C18PEG), each consisting of a lipophilic stearyl chain and a hydrophilic polyethylene glycol (PEG) polymer connected through a 1O2cleavable linker (Figure 1). We demonstrate that photochemical oxidation of these

polymers

subsequent

results

bond

in

cleavage

micellar

disruption

of

linker

the

due

between

to the

the

oxidation

and

hydrophobic

and

hydrophilic segments of the polymers, yielding responsive assemblies. These polymers

provided

a

photochemically-activated

therapeutic

platform

that

combines chemotherapeutic payload release with photodynamic therapy.


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Figure 1. a) Photooxidation and cleavage reaction of the amphiphilic polymer 9,10-C18PEG. b) Schematic depiction of micelle disruption upon anthracene photooxidation. c) Intracellular DOX release.

Experimental Section General Information. 1H and AVANCEIII

500

MHz

NMR,

13C

in

NMR spectra were acquired with a Bruker deuterated

solvent

at

room

temperature.

Chemical shifts are given in parts per million (ppm). All spectra were processed with Topspin 2.1 (Bruker Biospin) and further visualized with MestreNova, version 12 (Mestrelab Research, Santiago de Compostela, Spain). All reactions were monitored using silica gel 60 F254 analytical TLC plates with UV detection (λ = 254 nm and 365 nm). Silica gel (60 Å, 40-63 μm) was used

as

the

stationary

phase

for

column

chromatography.

The

spectrophotometric measurements of compounds were carried out in solvents of spectrophotometric quality. UV-vis absorption spectra were recorded using a Varian

Cary-100

spectrophotometer

in

double


beam

mode.

Irradiation

6

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experiments

were

performed

with

a

200

W

Hg/Xe

lamp

(Newport-Oriel)

equipped with a water filter, manual shutter, focusing lens, and the appropriate wavelength selecting filters. For irradiation at 365 nm, a combination of a 295 nm long-pass filter and a 365 nm band-pass filter was used, giving an average power density of 38 mW/cm2. For longer wavelength irradiations at  > 630 nm, a 630 nm long-pass filter was used, giving an average power density of 50 mW/cm2. For irradiation at  > 495 nm, two 495 nm long-pass filters were used, giving an average power density of 71 mW/cm2. Dynamic light scattering (DLS) measurements were performed on a Malvern Zetasizer Nano-ZS (Malvern Instrument Ltd., U.K.) equipped with a He−Ne laser operating at 633 nm at 25 °C. Samples were prepared in pure water and filtered through 0.2 μ m PTFE syringe filters before measurements. Mass spectrometry of polymers was performed using a Bruker Microflex MALDI-TOF mass spectrometer.


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High-resolution mass spectral (HRMS) analyses of new compounds were performed by the Massachusetts Institute of Technology mass spectrometry facility using electrospray ionization in positive mode. Transmission Electron Microscopy (TEM). Freshly prepared sample solution was drop-cast onto a TEM grid (carbon film, 400 mesh copper, Electron Microscopy Sciences), and the sample was allowed to dry at room temperature overnight. Negative staining was employed to enhance the imaging contrast.44 2% uranyl acetate solution was dropped on the sample for 30 seconds, following by drying using a filter paper. The structural of samples were inspected using a JEOL 2000FX TEM with an accelerating voltage of 200 kV. Chemicals. All starting materials and solvents were purchased from SigmaAldrich, TCI Chemicals, or Fisher Scientific and, unless otherwise specified, were used without further purification. Deuterated solvents were purchased from Cambridge Isotope Laboratories. mPEG-Amine and mPEG-OC18 (MW 2000 Da) were purchased from Creative PEGWorks. Doxorubicin, hydrochloride salt was purchased

from

LC

Laboratories.

9,10-dimethoxyanthracene,45


9-

8

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methoxyanthracene,46 cis-1,2-diphenyldithioethene47 were synthesized according to literature procedures. Results and Discussion A number of peroxides that result from cycloaddition of 1O2 are reported to cleave bonds in the original reactant; such moieties have the potential to be 1O

2-cleavable

groups in polymer materials. To inform designs of our

1O

2-

cleavable amphiphiles, we first compared the reactivities of a range of reported 1O

2-cleavable

groups by monitoring their disappearance in CH2Cl2 by UV-vis

spectrophotometry upon irradiation of the sensitizer methylene blue (Figure 2). We analyzed the kinetics of disappearance using a pseudo first-order kinetic model

for

the

methoxyanthracene,

disappearance

of

9,10-dimethoxyanthracene,

cis-1,2-diphenyldithioethene,

and

ethyl

β -dimethylamino

acrylate, all relative to a standard for chemical reactions with diphenylanthracene

(DPA).

The

Supporting

Information

9-

1O

2,

contains

9,10the

spectroscopic data for each kinetic trace (Figure S11). While the dithioethene and aminoacrylate each reacted with half the rate of DPA, the electron rich


9


Page 10 of 34

methoxyanthracenes reacted faster than DPA, with 9,10-dimethoxyanthracene reacting four times faster than 9-methoxyanthracene. These observations agree with the known trend that more electron rich, readily oxidized acenes react faster with 1O2 than less electron-rich acenes. To support the hypothesis that 1O2 is a key reactive oxygen species in these reactions of alkoxyacenes, we analyzed the products of photochemical oxidation of 9,10-dimethoxyanthracene under several conditions (See Figures S38-S42). Selective irradiation of methylene blue with  > 600 nm in the presence of 9,10-dimethoxyanthracene

in

acetone-d6

yielded

the

corresponding

endoperoxide, as would be expected upon cycloaddition with

1O

2

9,10-

with no

discernable byproducts by NMR spectroscopy. Especially key to making this assignment was the characteristic

13C

NMR signal for the bridgehead protons at

102.9 ppm. Similarly, direct irradiation of 9,10-dimethoxyanthracene at 365 nm yielded identical

1H

NMR and

13C

NMR spectra as selective irradiation of

methylene blue (Figure S38-S42). We therefore conclude that irradiation of


10

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9,10-dimethoxyanthracene

produces

1O

2,

which

then

undergoes

[4+2]

cycloaddition to yield the 9,10-endoperoxide.

Figure 2. Pseudo-first-order kinetics of reaction of four reported

1O

2-cleavable

groups upon irradiation of methylene blue in CH2Cl2; krel = 1 for 9,10diphenylanthracene.

We chose the two most reactive alkoxyanthracene derivatives as linking moieties in cleavable amphiphiles that contain PEG. In addition to being hydrophilic, PEG has the advantages of biocompatibility and resistance to fouling,48,49,50 as well the commercial availability of a variety of reactive derivatives. Scheme 1 summarizes our successful approach for synthesizing the


11


target

amphiphiles.

The

preparation

of

the

Page 12 of 34

9,10-dialkoxyanthracene-linked

amphiphile began with reductive alkylation of 9,10-anthraquinone. A mixture of

tert-butylbromoacetate and 1-bromooctadecane gave the desired unsymmetric product

1,

followed

deprotection

with

trifluoroacetic

acid

(TFA)

to

yield

carboxylic acid 2.51 Amidation of 2 with amine-functionalized PEG (2 kDa) under

standard

conditions

led

to

the

cleavable

amphiphile

9,10-C18PEG.

Following a similar strategy, reduction of 2-octadecyloxyanthraquinone52 gave the 3-alkoxy-9-anthrone regioisomer 3 selectively. Alkylation of 3 with tertbutylbromoacetate

yielded

the

desired

O-alkylated

compound

4.

After

deprotection with TFA, EDC-promoted amidation with methoxy PEG amine (2 kDa) again yielded target amphiphilic polymer 3,9-C18PEG.


12

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Scheme 1. Synthesis of the amphiphilic polymers 9,10-C18PEG and 3,9C18PEG, and chemical structure of commercially available, unreactive amphiphile C18PEG.

In each case, the molecular weight of PEG was 2 kDa.


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Figure 3. Left: Distribution of hydrodynamic diameters of 9,10-C18PEG and 3,9C18PEG micelles in water (1.0 mg/mL), determined by dynamic light scattering.

Right: Negative stain TEM images of 9,10-C18PEG and 3,9-C18PEG micelles (1.0 mg/mL and 0.5 mg/mL respectively).

Amphiphiles 9,10-C18PEG and 3,9-C18PEG appeared soluble in water by visual inspection, forming optically clear, foamy solutions upon gentle shaking and filtration through 0.2 μ m PTFE filters. Dynamic light scattering (DLS) of these samples revealed the presence of particles with average hydrodynamic diameters

of

~20

nm

for

both

polymers,

which

was

corroborated

by

transmission electron microscopy (TEM, Figure 3 and Figure S24-S28). While 9,10-C18PEG forms only spherical micelles (Figure S26), 3,9-C18PEG forms mixtures of spherical and rod-like micelles (Figure S27 and S28), with a larger proportion of rod-like micelles at higher amphiphile concentrations. Previous literature

indicates

that

spherical

micelles

aggregating

at

increasing

concentration of non-ionic PEG-based surfactants causes this sphere-to-rod transition.53 The sizes of these micelles were generally smaller than micelles


14

Page 15 of 34

prepared from block copolymers,38,39 and comparable to previously reported micelles comprising an n-C18H37 2

0.064 mM!

Absorbance

1.5

1

0.5

0 300

350

400 450 500 Wavelength (nm)

550

600

2 0.074 mM!

1.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.5

0 300

350

400 450 500 Wavelength (nm)

550

600

Figure 4. Sequences of spectra recorded over the course of titrating a 7.0 μM solution of eosin Y with a solution of 9,10-C18PEG (top) or 3,9-C18PEG (bottom): spectrum of eosin Y prior to the addition (thick black line); spectrum


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Page 16 of 34

after the addition of polymer (thick blue line). The inset shows titration profiles at 542 nm and determination of CMCs.

hydrophobic segment and PEG hydrophilic segment,54 indicating that the anthracene linkers do not perturb micellization. Changes in the absorbance of eosin Y as a function of amphiphile concentration enabled estimation of the critical micelle concentrations (CMCs) of these polymers,55 which we determined to be 64 M (0.16 mg/mL) for 9,10-C18PEG and 74 M (0.18 mg/mL) for 3,9C18PEG (Figure 4). These values are largely indistinguishable to the CMC of a commercially available analog (C18PEG) that does not include any anthracenes (89 M, Figure S13). Upon irradiation at 365 nm, the decomposition of the linkers triggered the disruption of the micelles. We monitored photochemical conversion of these samples by both UV/vis spectrophotometry and DLS in deionized water. The characteristic absorbance bands of the anthracene chromophores decreased upon irradiation, with the anthracene in 9,10-C18PEG reacting ~4-fold faster


16

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than that of 3,9-C18PEG (Figure S14 and S18), consistent with our earlier kinetics experiments in organic solvent. In addition, the optically transparent samples developed visible precipitate after photo-oxidation, which we ascribe to aggregates of hydrophobic photoproducts that contain anthraquinones, and in the case of 9,10-C18PEG, stearyl alcohol. The large and polydisperse nature of these aggregates made analysis of these “as-irradiated” samples by DLS impossible. After filtration, however, analysis of these samples by DLS showed partial photo-induced transformation of these small micelles into larger particles of average diameter between 100-200 nm, with the ~20 nm-diameter micelles of 9,10-C18PEG disappearing completely after one hour, while micelles of 3,9C18PEG were only partially consumed after two hours (Figure S15 and S19). To establish the products of UV irradiation of these anthracene-containing amphiphiles when they are assembled into micelles, we irradiated micellar aqueous solutions of 9,10-C18PEG or 3,9-C18PEG with λ = 365 nm, followed by lyophilization and

1H

NMR analysis of the samples in CDCl3, which dissolves

all photoproducts. Inspection of the aromatic region of the spectra revealed


17


9,10-anthraquinone

(Figure

5b)

as

the

primary

Page 18 of 34

aromatic

product

of

the

irradiation of 9,10-C18PEG (Figure 5a), and 2-octadecyloxyanthraquinone (Figure 5d) as the primary aromatic product of the irradiation of 3,9-C18PEG (Figure 5c). These anthraquinone products are consistent with the proposed mechanism of photooxidation and cleavage of alkoxyacenes reported in the literature.29,56 We therefore conclude that the bonds between the hydrophobic and hydrophilic segments of these amphiphiles cleave upon irradiation due to exposure to 1O2, which leads to micelle disruption.


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Figure 5. Aromatic region of

1H

NMR spectra in CDCl3 of a) 9,10-C18PEG

before and b) after one hour UV irradiation and c) 3,9-C18PEG before and d) after 12 hours UV irradiation.

In an initial demonstration of phototriggered release, we loaded micelles comprising 9,10-C18PEG, 3,9-C18PEG, or C18PEG with the guest doxorubicin (DOX), a hydrophobic chemotherapeutic drug.57,58,59 The release of DOX from the solubilizing environment of the micelle was monitored by the decrease of intensity of the DOX band in the UV-vis spectra of an aqueous solution of micelles loaded with solubilized DOX after irradiation ( λ

= 365 nm), and

filtration with a 0.2 µm PTFE syringe filter to remove the de-solubilized DOX that aggregates upon micelle disruption. We ascribe the faster rate of DOX removal from 9,10-C18PEG micelles compared to 3,9-C18PEG and C18PEG micelles (Figure 6 and Figure S33-S35) to faster decomposition of the 9,10dialkoxy cleavable linkers.


19


Page 20 of 34

Figure 6. Release of DOX from micelles in water upon exposure to light ( λ = 365 nm) and in the dark at room temperature. The amount of DOX removed from the solubilizing environment of the micellar cores upon irradiation of 9,10C18PEG micelles is greater than the amount released from 3,9-C18PEG and C18PEG micelles. Error bars show standard errors of three replicates.

A potential advantage of 1O2-responsive materials is the range of sensitizers that can produce

1O

2

upon irradiation with visible or near-infrared light. We

used eosin Y as a photosensitizer, as Methylene Blue gave rise to larger aggregates according to DLS measurements, while Rose Bengal exhibited poor photostability. Irradiation of a solution containing both eosin Y and 9,10-C18PEG


20

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at λ > 495 nm yielded irreversible oxidation of the anthracene linker (Figure S16). DLS analysis after filtration did reveal partial transformation of the micelles into larger particles (Figure S17), but micelles were not completely consumed after one-hour irradiation as observed during exposure to UV light. Irradiation at λ > 495 nm in the absence of sensitizer yielded no anthracene oxidation (Figure S22). In addition, removal of O2 from the micellar suspension slowed reaction of anthracene in 9,10-C18PEG dramatically, achieving only 2025% conversion after 60 minutes (Figure S23); similar conversion in aerated water required less than 3 minutes (Figure S16). To evaluate their therapeutic potential in vitro, we compared the extent to which these micelles reduced the viability of HeLa cells (Figure 7) when they were: i) either devoid of cargo or loaded with DOX, and ii) either kept in the dark or irradiated at 365 nm. Confocal microscopy of DOX-loaded micelles, using the intrinsic fluorescence of DOX as an imaging agent, confirmed cellular uptake of DOX loaded micelles into the cytosol of HeLa cells (Figure S37). Micelles comprising the control amphiphile C18PEG, and therefore lacking any


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moiety that absorbs 365 nm showed minimal toxicity, regardless of whether they were irradiated, loaded with DOX, or both. In contrast, irradiation of micelles comprising either 9,10-C18PEG or 3,9-C18PEG, but not loaded with DOX, decreased cell viability to 44-56%, while identical samples kept in the dark did not show increased toxicity relative to the control amphiphile C18PEG. We attribute this photoinduced cytotoxicity in the absence of DOX to 1O2, and perhaps

other

ROS,

formed

upon

direct

irradiation

of

alkoxyacene

chromophores. When loaded with DOX and irradiated with light, micelles comprising 9,10-C18PEG showed even greater toxicity by reducing cell viability to 27±3%, while the same sample in the absence of light showed minimal toxicity.

We

ascribe

the

increased

phototoxicity

of

9,10-C18PEG

micelles

compared to 3,9-C18PEG micelles to faster decomposition of the 9,10-dialkoxy cleavable linkers, yielding faster release of the DOX cargo, in accordance with the model release experiments (Figure 6). These results are consistent with cooperative photoinduced cytotoxicity of drug-loaded 9,10-C18PEG micelles that combine the traditional photodynamic effect of ROS production with photo-


22

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triggered release of DOX. The estimated concentration of DOX used for in vitro delivery experiments (0.18 M) is lower than the IC50 (50% of growth inhibition) that we measured for doxorubicin acting on HeLa cells (1.3 M), which is in accordance

with

the

reported

IC50

(1-5

M),60,61

further

suggesting

the

combination of ROS and released DOX as responsible for the increased cytotoxicity.

Figure 7. Cell viability of HeLa cells upon exposure to micelles that are either: i) loaded with DOX (YES) or not loaded (No), and ii) irradiated at 365 nm in the presence of the cells (Light) or not irradiated (Dark). Error bars show


23


Page 24 of 34

standard deviations of four replicates. The irradiation time for all “light” experiments was 60 minutes.

Conclusions We have demonstrated the rational design of nanometric, photo-oxidatively degradable micelles containing alkoxyanthracene-based singlet oxygen cleavable groups,

including

a

9-alkoxy-10-hydroanthracene

linker.

The

products

of

irradiation of these micelles in aqueous solution are consistent with reaction with singlet oxygen to form endoperoxides, followed by bond cleavage to yield the corresponding quinones. The reactivity of the anthracene moieties controls the kinetics of micelle degradation, the rate of cargo release, and the observed photoinduced cytotoxicity. From a materials design perspective, our results provide a flexible platform for the discovery of nanoscale delivery vehicles that exhibit

enhanced

photodynamic

therapy—the

combination

of

photodynamic

therapy with photo-responsive materials that can release a trapped drug—while laying the groundwork for potential paths forward for harnessing visible or NIR light through either the integration of a separate sensitizer or modifications of


24

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alkoxyacenes to absorb longer wavelengths. More fundamentally, this work also highlights the value of continuously improving our understanding of chemical reactivity for the rational design of responsive nanomaterials.

ASSOCIATED CONTENT

Supporting Information.

The following files are available free of charge. 1H

and

13C

NMR spectra, MALDI-TOF mass spectra, UV-Vis measurements,

Irradiation experiments, Control experiments, DOX loading and release experiments, Cell viability, Cellular uptake, in vitro irradiation experiments (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (S.W.T.)


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ACKNOWLEDGMENT

Funding from the NIH (EB022207) supported the structure-property studies and model release experiments, while funding from the NSF (CHE-1609146, to ST) and the NIH (EB02264, to VR) supported the fluorescence microscopy, transmission electron microscopy, and in vitro experiments. We thank James Chambers (Light Microscopy Facility at UMass Amherst) for assistance with confocal laser scanning microscopy.

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Polymer Amphiphiles for Photoregulated Anticancer Drug Delivery

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